Traditionally, spring cores for mattresses have consisted of a plurality of spaced parallel rows of helical coil springs mounted between border wires; coil springs adjacent the border wires being attached thereto via helical lacing wires, sheet metal clips or other connectors. The upper and lower end turns of adjacent coil springs are generally connected to each other by helical lacing wires. Coil springs are arranged in longitudinally extending columns and transversely extending rows. Padding and upholstery commonly are secured to opposed surfaces of the spring core, thereby resulting in what is known in the industry as a two-sided mattress for use on either side.
Recently, spring cores have been developed having only one border wire to which the end turns of the outermost coil springs are secured. After padding and/or other materials are placed over the upper surface of the spring core in which the border wire is located, an upholstered covering is sewn or secured around the spring core and cushioning materials, thereby creating what is known in the industry as a one-sided or single-sided mattress.
The upper and lower end turns of unknotted coil springs often are made with straight portions or legs which abut one another when coil springs are placed next to each other. For example in U.S. Pat. No. 4,726,572, the unknotted end turns of the coil springs have relatively straight legs of an identical length. Adjacent coil springs are connected to each other at their end turns with helical lacing wire. One leg of an end turn of a coil spring is set beside the opposite leg of an end turn of the adjacent coil spring. The side-by-side legs are laced together with helical lacing wire.
When assembled, coil springs of such a spring core may move within the helical lacing wire, causing misalignment or nonparallel alignment of coils in adjacent rows of coils. This misalignment causes the coil springs to line up improperly. The lines connecting the central axes of the coil springs no longer form a 90 degree angle as they should. This misalignment changes a rectangular or square spring core into a rhombus. Such an odd shape must then be corrected at additional cost. This will, in most cases, result in compression problems when a spring unit is compressed for shipping purposes. Misaligned coils will be damaged in the forced compression/decompression. In a mattress construction, wrongly compressed coils will result in an uneven sleep surface. This uneven sleep surface will be visible to a consumer after the cushioning materials, such as foam and fibrous materials take their set normally after a few months of use.
In order to avoid this misalignment problem, spring cores have been developed having individual coil springs with U-shaped end turns having one leg of a greater length than its opposing leg, as in U.S. Pat. No. 4,817,924. Once again, adjacent coil springs of the spring core of U.S. Pat. No. 4,817,924 are connected with helical lacing wire at their end turns. However, due to the difference in leg lengths of the U-shaped end turns, the helical lacing wire wraps one more revolution around the longer leg of the U-shaped end turn than around the shorter leg of the U-shaped end turn of the adjacent coil spring. The different leg lengths bound together with helical lacing wire corrects the misalignment or coil offset situation.
Coil springs with unknotted end turns, such as those disclosed in U.S. Pat. Nos. 5,584,083 and 4,817,924, have upper and lower end turns which are rotated approximately 180 degrees in relation to each other to dispose the shorter and longer legs of the upper end turn in mirror symmetry to the shorter and longer legs, respectively, of the associated lower end turn. Such an orientation eases the manufacturing process by allowing all the coil springs of the spring core to be oriented in an identical manner except for one outermost row (or column) of coil springs, the coil springs of which are rotated relative to the remainder of the coil springs in order to enable the end turns of all of the coil springs to be secured to the border wires. The identical orientation of the coil springs (except for the one row or column) allows the long leg of an end turn of one coil spring to be helically laced with the shorter leg of the end turn of the adjacent coil spring for reasons described above.
One drawback to a spring core assembled in such a manner is that the coil springs may exhibit a pronounced tendency to incline laterally away from the open end of the end turn when a load is placed on them. One solution which has been utilized to overcome this leaning tendency has been to orient the coil springs having unknotted end turns in a checkerboard fashion within the spring core, every other coil spring within a particular row or column being twisted 180 degrees so the free end of the end turns are helically laced together, as shown in U.S. Pat. No. 6,375,169. However, to align the coil sprigs in such a checkerboard manner may be difficult to do on an automated machine, time consuming and therefore expensive.
In order to reduce the coil count of a spring core (the number of coil springs used in a particular sized product) and therefore, the expense of the spring core, it may be desirable to incorporate into the spring core coil springs having unknotted end turns which are substantially larger than the diameter of the middle or central spiral portion of the coil spring. Prior to the present invention, such coil springs exhibited exaggerated lean tendencies, i.e. the greater the head size or size of the end turns, the greater the lean when a load was placed on the coil spring.
Therefore, there is a need for an unknotted coil spring which does not lean or deflect in one direction when loaded.
The greatest expense in manufacturing spring cores or assemblies is the cost of the raw material, the cost of the steel used to make the coil springs which are assembled together. Currently, and for many years, the wire from which unknotted coil springs have been manufactured has a tensile strength no greater than 290,000 psi. This standard wire, otherwise known as AC&K (Automatic Coiling and Knotting) grade wire has a tensile strength on the order of 220,000 to 260,000 and is thicker, i.e. has a greater diameter, than high tensile strength wire, i.e. wire having a tensile strength greater than 290,000 psi. In order to achieve the same resiliency or bounce back, a coil spring made of standard gauge wire must have one half an additional turn when compared to a coil spring made of high tensile wire. In other words, the pitch of the coil springs made of high tensile wire may be greater as compared to coil springs made of standard wire. Coil springs made of high tensile strength wire also do not tend to set or permanently deform when placed under significant load for an extended period of time, i.e. during shipping. Therefore, there is a desire in the industry to make coil springs having unknotted end turns of high tensile strength wire because less wire is necessary to manufacture each coil spring.
Although coil springs made of high tensile strength wire may be desirable for the reasons stated above, coil springs made of wire having too high a tensile strength are too brittle and may easily shatter or break. Therefore, there is a window of desirable tensile strength of the wire used to make coil springs having unknotted end turns.